Known dynamic random access memory (DRAM) devices include a switching transistor and an integrated storage capacitor tied to the storage node of the transistor. Incorporating a stacked capacitor or a trench capacitor in parallel with the depletion capacitance of the floating storage node enhances charge storage. Due to a finite charge leakage across the depletion layer, the capacitor is frequently recharged or refreshed to ensure data integrity in the DRAM device. Thus, such a DRAM device is volatile. A power failure causes permanent data loss in a DRAM device. DRAM devices are relatively inexpensive, power efficient, and fast compared to non-volatile random access memory (NVRAM) devices.
A minimum capacitance per cell is required to sense a conventional DRAM cell. A significant challenge for every succeeding generation of reduced feature size is to provide this minimum capacitance per cell. A memory cell design goal is to achieve an 8F2 DRAM cell. To that end, complex three-dimensional capacitor structures have been designed. However, these complex three-dimensional capacitor structures are difficult to manufacture and adversely impact yield. There has been serious concern of the scalability of the conventional DRAM cell beyond the 0.1 μm lithographic generation. The scaling problems have been aggravated by increased device short channel effects and leakages associated with complicated capacitor structures. Thus, the elimination of the stacked capacitor or trench capacitor in a DRAM cell is desirable.
A silicon-on-insulator (SOI) capacitor-less single-transistor DRAM cell has been proposed by S. Okhonin et al. The state of the floating body charge in the transistor affects the channel conductance of the transistor and defines the memory state (“1” or “0”) of the cell. Two methods for generating carriers in the body were proposed. The generated carriers are holes for the partially depleted (PD) SOI-NFET or electrons for the PD-SOI-PFET. One proposed method generates carriers using the drain-edge high field effect associated with impact ionization. In another case, the carriers are generated by the parasitic bipolar phenomenon.
The memory retention for these SOI capacitor-less single-transistor DRAM cells depends on the device channel length. That is, the stored charge retention time decreases with decreasing channel length. Additionally, the memory retention depends on recombination charge constants and multiple recombination mechanisms, and thus is expected to be both temperature and process sensitive. Therefore, controlling the memory retention between refresh operations is expected to be difficult.
Known non-volatile random access memory (NVRAM), such as Flash, EPROM, EEPROM, etc., store charge using a floating gate or a floating plate. Charge trapping centers and associated potential wells are created by forming nano-particles of metals or semiconductors in a large band gap insulating matrix, or by forming nano-layers of metal, semiconductor or a small band gap insulator that interface with one or more large band gap insulating layers. The floating plate or gate can be formed as an integral part of the gate insulator stack of the switching transistor.
Field emission across the surrounding insulator causes the stored charge to leak. The stored charge leakage from the floating plate or floating gate is negligible for non-volatile memory devices because of the high band gap insulator. For example, silicon dioxide (SiO2) has a 9 ev band gap, and oxide-nitride-oxide (ONO) and other insulators have a band gap in the range of 4.5 ev to 9 ev. Thus, the memory device retains stored data throughout a device's lifetime.
However, there are problems associated with NVRAM devices. The writing process, also referred to as “write-erase programming,” for non-volatile memory is slow and energy inefficient, and requires complex high voltage circuitry for generating and routing high voltage. Additionally, the write-erase programming for non-volatile memory involves high-field phenomena (hot carrier or field emission) that degrades the surrounding insulator. The degradation of the insulator eventually causes significant leakage of the stored charge. Thus, the high-field phenomena negatively affects the endurance (the number of write/erase cycles) of the NVRAM devices. The number of cycles of writing and erasing is typically limited to 1E6 cycles. Therefore, the available applications for these known NVRAM devices is limited.
Floating plate non-volatile memory devices have been designed that use a gate insulator stack with silicon-rich insulators. In these devices, injected charges (electrons or holes) are trapped and retained in local quantum wells provided by nano-particles of silicon embedded in a matrix of a high band gap insulator (also referred to as a “trapless” or “limited trap” insulator) such as silicon dioxide (SiO2) or silicon nitride (Si3N4). In addition to silicon trapping centers, other trapping centers include tungsten particles embedded in SiO2, gold particles embedded in SiO2, and a tungsten oxide layer embedded in SiO2.
There is a need in the art to provide dense and high speed capacitor-less memory cells with data non-volatility similar to Flash devices and DRAM-like endurance as provided by the present subject matter.